The present invention relates to an ultrasound probe and a method of producing the same and in particular to an ultrasound probe comprising a plurality of layered inorganic and organic piezoelectric elements layered on each other and a method of producing the same.
Conventionally, ultrasound diagnostic apparatuses using ultrasound images have been employed in the medical field. Generally, an ultrasound diagnostic apparatus of this type transmits an ultrasonic beam from an ultrasound probe into a subject's body, receives the echo from the subject with the ultrasound probe, and electrically processes the resulting reception signals to produce an ultrasound image.
In recent years, it has been a mainstream to utilize harmonic imaging where a harmonic component, which is generated as the ultrasonic waveform deforms due to non-linearity of the subject, is received and visualized to give more accurate diagnosis. In addition, as a new diagnostic method using ultrasonic waves, attention has been paid to photoacoustic imaging in which a living body is irradiated with a laser, whereby a weak broadband elastic wave is generated due to adiabatic expansion to be received and visualized.
Ultrasound probes appropriate for use in harmonic imaging or photoacoustic imaging have been proposed, an example of which is disclosed by WO 2008/010509 and is formed of a plurality of inorganic piezoelectric elements using inorganic piezoelectric bodies made of lead zirconate titanate (Pb(Zr, Ti)03) or the like and a plurality of organic piezoelectric elements using organic piezoelectric bodies made of polyvinylidene fluoride (PVDF) or the like layered over each other.
The inorganic piezoelectric elements can transmit higher power ultrasonic beams, and the organic piezoelectric elements can sensitively receive harmonic signals. In addition, the inorganic piezoelectric elements can acquire a reception signal of an ordinary ultrasonic wave, and the organic piezoelectric elements can sensitively receive a broadband signal in photoacoustic imaging.
Ultrasonic beams outputted from the inorganic piezoelectric elements penetrate organic piezoelectric bodies and thereafter are emitted from the ultrasound probe into the subject's body. Hence, the thickness of the organic piezoelectric bodies is designed so as to improve the acoustic transmittance of the ultrasonic beams. In particular, the organic piezoelectric bodies are designed to have a thickness which is near the value satisfying the λ/4 resonance condition when the fundamental waves from the inorganic piezoelectric elements have a wavelength λ. Accordingly, the organic piezoelectric bodies cannot conventionally be freely designed to have an arbitrary thickness but had to be designed to have a certain thickness in order to satisfy the above-described resonance condition. On the other hand, since the organic piezoelectric body has a small relative permittivity, if the organic piezoelectric elements are formed to be thick, the electrical capacitance thereof becomes small. Hence, it has been difficult for a circuit to efficiently acquire a reception signal generated by an ultrasonic wave received with the organic piezoelectric elements. Moreover, since thermal noise becomes large if the electrical capacitance is small, a signal-to-noise ratio with respect to the obtained received signal tends to be disadvantageous.
In a case where the organic piezoelectric element is laminated on the inorganic piezoelectric element, focus misalignment or decrease in reception efficiency will occur unless the positions of their electrodes coincide with each other with respect to the direction of beam transmission. Accordingly, it is preferable that positions of electrodes of the inorganic piezoelectric element and of the organic piezoelectric element laminated on the inorganic piezoelectric element coincide with each other in the direction of beam transmission as much as possible. However, it has been difficult to achieve precise coincidence in a conventional configuration or a conventional method of producing an ultrasound probe.
Furthermore, as the temperature of the organic piezoelectric body increases, crystallinity thereof gradually decreases. Hence, the upper limit temperature for use is considerably lower than the Curie point. As typical examples, polyvinylidene fluoride (PVDF) has an upper limit temperature for use of 80° C., and a polyvinylidene fluoride trifluoroethylene copolymer (P(VDF-TrFE)) has an upper limit temperature for use of 100° C. Accordingly, if the materials are heated to a higher temperature than such temperatures during processing, ferroelectricity deteriorates, and depolarization occurs. One effective means for recovering the deteriorated ferroelectricity is repolarization. However, the coercive electric field (Ec) of the organic piezoelectric body is extremely high, such as about 400 kV/cm to about 450 kV/cm. Therefore, application of an extremely high voltage is required in order to cause, on the device, repolarization of the organic piezoelectric body that has once undergone depolarization, but application of such a high voltage is difficult from a standpoint of processing. Accordingly, when the organic piezoelectric body is laminated on the inorganic piezoelectric body, they need to be processed at the lowest possible temperature with the minimum possible heat history. However, processing with almost no heat history has been difficult in a conventional configuration or a conventional method of producing an ultrasound probe.